A. Field of Invention
This invention pertains to a method and apparatus for the manufacture of a toothed power transmission belt, and more particularly to a means of determining the nominal position of each tooth in a molded belt prior to the process of molding the tooth.
B. Description of the Related Art
Synchronous power transmission belts transmit motion and power by the engagement of teeth on the belt to teeth on driving and driven sprockets. Commercial tolerances for some of these belts are given in RMA Engineering Standards IP-24 and IP-27. The tolerance of pitch length on such belts ranges from 4444 ppm for miniature belts of 91.44 mm pitch length to 291 .5 ppm for large belts of 6860 mm pitch length. IP-27 specifies a pitch length tolerance of +xe2x88x921.12 mm for a 200 tooth, 14 mm pitch belt, so the average tooth pitch tolerance is xc2x10.0056 mm. The pitch between adjacent teeth must be generally within xc2x10.0254 mm of the nominal pitch, and must be statistically distributed about the nominal value.
Some synchronous belts are made by applying layers of suitable belt materials to the circumference of a cylindrical mold. The mold is made with accurately spaced cavities on the internal or external circumference to form and vulcanize all of the teeth on the belt simultaneously. The spacing of teeth on the finished belt is largely dependent on the spacing accuracy of the mold cavities. The number of teeth in the belt must equal the number of cavities in the mold, so a different mold is required for each number of belt teeth and tooth spacing.
An alternative process uses a curved or flat mold with accurately spaced tooth forming cavities to mold and vulcanize an incremental portion of the belt teeth. The process is repeated in overlapping or abutting sections until all belt teeth are complete. Incremental molding with a flat mold is often called press cure. Incremental molding with a curved mold is often called roto-cure. Statistical variation of material properties, thermal expansion, and dimensional variation of uncured belt components often result in cumulative error of pitch and tooth position when a synchronous drive belt is cured incrementally. This cumulative error can result in excessive pitch error between molded sections, especially when a leading portion of the last section is cured to a trailing portion of the first section.
Press cured power transmission belts usually contain layers of fabric and elastomer that are formed into a flexible cylinder. A helical layer of one or more tensile members is also usually included. In a typical manufacturing process, the uncured and unmolded belts are supported on cylinders placed on either side of a planar mold. The cylinders, which may include grooves or ridges on their circumference, maintain alignment of the belts to the mold during the sequential molding of belt sections, and also apply tension to the cords of the belt to prevent their displacement in the molding process.
Roto-cured power transmission belts also contain layers of fabric and elastomer that are formed into a flexible cylinder. A helical layer of one or more tensile members is also usually included. In a typical manufacturing process, the uncured and unmolded belts are supported on cylinders, with at least one of the cylinders having tooth-forming cavities on the cylinder circumference. A concave outer mold is pressed against the outside of the belt to form the back side. The outer mold may itself be a flexible belt or band, which is wrapped around the supporting cylinder mold and belt. The cylinders maintain alignment of the belts during the sequential molding of belt sections, and also apply tension to the cords of the belt to prevent their displacement in the molding process.
The spacing of the teeth on the molded belt must be even all around the belt, even when the belt is shorter or longer than the nominal circumference. The exact pitch length circumference of the uncured belt is difficult to measure accurately because the radius of the belt neutral plane on the cylinders is difficult to determine.
The present invention provides methods and apparatuses for achieving uniform tooth spacing independent of the actual circumference of the belt.
The present invention is directed to a new and improved method of controlling the pitch of adjacent teeth in a synchronous power transmission belt. The improved method is particularly useful for cylindrical belts that are sequentially cured in sections in a planar press.
In accordance with the present invention, there is provided a method for manufacturing a toothed belt on a manufacturing machine, the manufacturing machine having a first cylinder, a second cylinder, and a planar mold, the toothed belt having an integral number of teeth and a design length at a certain tension. The inventive method comprises the steps of:
marking a first location on an uncured belt with a first mark;
determining a circumference of the uncured belt;
marking a second location on the uncured belt with a second mark to identify a first section of the uncured belt to be molded;
identifying sequential sections of the uncured belt to be molded;
marking a first location at a front effective edge of the mold with a first mold mark;
marking a second location at a rear effective edge of the mold with a second mold mark;
determining an actual length of the first section;
determining a working length of the mold;
calculating a first ratio of the actual length of the first section to the circumference of the uncured belt;
determining a number of teeth to be molded into the first section by multiplying the first ratio by a total number of teeth to be molded into the uncured belt;
calculating a second ratio of the number of teeth to be molded into the first section to a number of tooth-forming cavities in the mold;
calculating a required length of the first section by multiplying the working length of the mold by the second ratio;
adjusting a position of one of the cylinders to change the actual length of the first section to the required length of the first section;
positioning the uncured belt relative to the mold so that the first mark on the uncured belt is a first predetermined distance from the first mold mark and the second mark on the uncured belt is a second predetermined distance from the second mold mark;
applying the mold to the uncured belt to form teeth in the first section;
sequentially repositioning the uncured belt to mold the sequential belt sections.
One advantage of this invention is that cumulative error is eliminated by positioning each section of teeth relative to the underlying belt structure and not relative to the previous section of the belt that was just cured. The previous section of the belt that was just cured might have position errors within its own position tolerance and these errors, if allowed to accumulate, can make the belt tooth pitch exceed specification tolerances.